I found out today that our electric co-op will start implementing "time of use" rates.  Per our co-op, it will work as follows.  Power will be available 24/7 without interruption.  The non-peak rate will be 7.6 cents per kWh.  Peak rates will be 17.0 cents per kWh.  Peak times are from 6:00am-8:00am, and from 3:00pm-8:00pm.

I would like to maximize savings with our water to air geothermal system as much as possible without being too uncomfortable (I know comfort is subjective) and without spending much, if any, money on storage.  We have a programmable thermostat with 4 program settings per day.  My initial thought is to use some of the program settings to make the house temperature a few degrees colder when in cooling mode, and a few degrees warmer when in heating mode, just before peak times, to "ride out" most, if not all, of the peak time.  I also know it will be more uncomfortable inside during these times when outside temperatures are extreme.

Is there anything else we can do other than what I have mentioned?

Thanks!

P.S.  Domestic hot water heating is not an issue since we have a large tank with low heat loss that can carry us through peak times without power (we have a water heater timer).

Just make sure to avoid aux strip use during the pre-peak heating runup. (and post peak recovery)

Success will somewhat depend on thermal mass of house and possibly solar gain in a key room or two.

Years ago I moved my central air in a Philly townhouse to an interruptible meter. On really hot days I'd drop the setpoint to 68 or so in the morning before power went out from 1 to 6 PM. It worked reasonably well, though I dimly recall getting my first lesson in the whys and wherefores of an iced evaporator

If you have an electric clothes dryer be sure to minimize its use during peak period

Thanks Curt. I'll keep the aux temperature lockout set and be wary when it is cold outside. I'll start with 2f and see how that does. Propane dryer. Will still do laundry off-peak. The wife didn't like my idea of pre-made toast on a 15 amp appliance timer.

I grew up in Philly. See the video of a cop using a taser on a Phillies fan that ran on field yesterday. That'll teach him. Heard the kid's mother apologized. Duh. LoL

> 7.6 cents per kWh. Peak 17.0 cents per kWh. Peak times are from 6:00am-8:00am, and from 3:00pm-8:00pm

Those are very attractive rates and times. A water output heat pump would just require a tank to eliminate all peak usage. And not all that large with those times.

Could possibly be worth it to get a scrap water-air heat exchanger and pull heat out of a larger water heater tank during peak times - ie, use the desuperheater to heat water for later space heating use.

Please check this reasoning as I haven't had morning coffee yet. 17 cents compared to 7.6 cents is about 2.24 times. So, as long as I don't run the geothermal system 1.24 times MORE in non-peak time (than I would have otherwise run it during peak time) we'd be ahead from a cost standpoint.

There are probably too many variables to figure the following without monitoring equipment, but just wondering if this would work in theory. Assuming the above is correct, would I actually have anything close to 1.24 times more run time by adjusting the thermostat temperature a few degrees (let's say 2f) colder when in cooling mode? Using cooling as an example, our current set point is 75f. So, I would make it 73f just before peak time begins. I doubt my wife would allow the temperature to swing above 75f, so there is no corresponding cost offset by letting the temperature get to 77f. Non-peak electricity pricing will be the only offset to overall cost.

What do you think, other than I should get that coffee. :-)

The more I think about this strategy (post coffee), the more I think I'm wrong from a cost reduction standpoint. I can foresee the additional run time during a pre-peak ramp-up offsetting potential savings too much. Probably best to just cut back as much as possible during peak times, then restore settings to "normal" when off-peak.

Glad we just finished our first year with geothermal for a cost comparison. This rate structure change will throw future cost comparisons, to year 1, out the window.

Thanks jonr too. Interesting idea.

Post-beer, I disagree. Given the large cost difference between rates any loss of efficiency (which, IMO, would be slight) via long runtimes pre and post peak in order to alternately bank btus in the structure pre peak as well as recover post-peak would be overwhelmed by the savings by avoiding peak.

If wife allows, configure a 2 degree 'setup' (more heat in winter, more cool in summer) for the 2 hours pre peak. Then configure a 2 degree setback during peak, then configure normal setpoint post peak. Just keep aux strips out of the picture.

Owing to the very low rate of heat recovery by a desuperheater (just 5-10% of unit total capacity) I do not share Jonr's enthusiasm for a separate storage and transfer arrangement based upon it. At best it might put away a few thousand btus per day, just pennies, and the arrangement would steal from the desuper's normal role of preheating domestic hot water. The first cost, control complexity, and physical volume all militate against such a system.

Let's not misrepresent "could possibly be worth it" as enthusiasm - in fact, let's call it skeptical. The timing suggests a 30% deficit in heat output. The set up/set down idea might save 15%. That leaves 15% which would mostly be covered by a desuperheater producing 10% (minus hot water usage) and completely by a larger desuperheater.

> At best it might put away a few thousand btus per day

Let's do the math and say over 100,000 at peak loads. But I'm still skeptical that it makes sense, especially with a normally sized desuperheater.

Posted By jonr on 06 May 2010 08:07 AM
...especially with a normally sized desuperheater

Any suggestions on how to supersize a desuperheater?
...or how to supersize the available BTUs of superheat?


If only we had some bacon, we could have bacon & eggs,

...if only we had some eggs,

Looby

If one thinks that a desuperheater can only extract superheat, they have a lot to learn.

You can lead a horse to water, but you can't make him drink.

Looby, I'm getting hungry! 

Curt, I tried to attach an Excel spreadsheet to this post, but file type is not allowed.  Here is an image instead.  I used an example to illustrate your point, with some success.  Please let me know if you, or anyone, think this needs adjustment.

Cost seemed to confuse things, so what I did was to calculate that for every 1 hour of run time during peak time, I could have run 2.24 hours of non-peak time for the same cost, (based on 17 cents divided by 7.6 cents.)  I'll call this figure "equivalent non-peak hours".

Without modifying the thermostat schedule at all, there is 1.75 hours of real run time (or 3.3 hours equivalent non-peak hours.)
Modifying the thermostat settings to all non-peak hours, and keeping 1.75 hours of real run time, yields a savings of 47%.
Does this make more sense?

We have been using on peak / off peak for over 20 years now. What I can add is dropping or raising the temperature further out will tend to carry the space better. What I have found is if I can get the house from 75F to 73F in 2 hours, that’s fine. But if I set it to go to 73F five hours before on peak it gets to the point it is cycling at 73F it will stay there much longer. The conclusion I came to was in dropping the temp (or warming) the air will get warmer or cooler, but everything in it with mass didn’t warm or cool as much, maybe one degree in that direction, and in trying to reduce swing mass seems to be what matters most. So you might be better off dropping from 75F to 74F for five hours compared to 75F to 73F for the last two hours.

There are other tricks you can do as well. For example I have our chest freezer on an appliance timer that shuts it off for the last three hours of on peak. I played with it quite a bit watching the temps and found I could go about 5 hours and still be below 20F, but thought three hours was a good compromise.

I also did the same thing with the fridge and found about one hour worked fine. Unfortunately my wife has put a stop to that. The problem is she can't get ice and water from the door and it drives her nuts. I thought about breaking just the lead to the compressor, but didn't want to mess with her new fridge. I also manually turn on the ice maker on weekends (all off peak) and rarely have to run it during the week.

I suppose one could increase air flow (ceiling fans?) to simultaneous make an overheat period equivalent in comfort and increase convection/transfers to/from the interior mass.

Thanks Brock! I'll experiment with thermostat temperatures and times to see what works best for our house. What is the structure of your house? Ours is a conventional stick frame. Not much thermal mass, but every little bit should help.

Funny, I thought of the fridge on a timer. Yesterday I checked the USDA web site (link not appearing?).  I had all but talked myself out of the idea, but since you mentioned it, I'll do some experimenting, making sure not to exceed the USDA time/temperature guidelines. We have one fridge in the garage (not the best place, but it's more convenient than the basement), and one in the kitchen (who would have thought!) I'll first make sure the temperatures are set to guidelines before experimenting. My wife will probably not like the light not functioning. :-)

Any other tricks you can think of? So far I have thermostat settings, washing and drying clothes, adjusting TV viewing and 200 watt/ch stereo amplifier usage, vacuuming, dishwasher, and water heater. We don't use much ice, but since the tray is in the freezer, I will fill it up with ice to help maintain freezer temperature (more mass). CFL bulbs are already throughout the house. Surprised that hasn't landed me in divorce court yet.

Guess timing of the coffee maker will help too. Maybe the wife will wake up an hour earlier so hair can be blow dried off peak.

jonr, yep.  I may increase the ceiling fan speed too.

P.S.  My wife said it's either sleep more and use the hair dryer during peak rates, or wake up early and use the coffee machine and the dryer off-peak!

Geome - the spreadsheet makes sense.

Brock - I concur - longer prepeak setup / setback makes sense - more time for delta-T to 'soak' into building's thermal mass, rather than just the air and lightweight mass that are affected in just an hour or so.

Ignore coffee maker and blow dryer loads owing to short duration

Our house is also stick frame with spray foam. And yes watch those fridge temps. You should easily get an hour out of it, but like you mentioned you're more likely to run in to a no light problem with your wife then temps to high

Any loads that are short term won't add up to much, unless your wife is blow-drying her hair for 30 minutes you should be fine. You already hit the big ones, dishwasher, washer, dryer, thermostat setbacks, water heater. Then you're in to the smaller stuff, like making ice at the start of off peak, adding room temp soda / beer to the fridge at off peak. The big things will matter more the smaller things are more nitpicky. When my wife uses the waffle maker or electric griddle on peak it sucks up the power from the last two weeks of my little things. I even go so far as to charge the kids hand held games and electric ride on toys off peak. The key is to do shift things that won't irritate my wife

Thanks guys. Yep, hair dryer and coffee maker (turned off after brewing) cycles are short.

Brock, did you use the timer on the refrigerator just 1 time per day? We could potentially turn off power from 7-8am and from 7-8pm. These are the last 1 hour periods of our peak times, and our timers will accommodate multiple on/off cycles per day. Good thought on putting warmer food in the refrigerator off peak.

Honeywell is supposed to have an update released for their Prestige thermostats that will include a feature for time of use rates. Not sure if this will be more useful than the 4 program settings per day that are already available. The update is also supposed to have a different auxiliary heat management function for geothermal systems.

Posted By geome on 04 May 2010 08:52 PM
...  Peak rates will be 17.0 cents per kWh.  Peak times are from 6:00am-8:00am, and from 3:00pm-8:00pm.

You should be able to ride out the morning rate spikes without any problems, but the evening rate spike is 5 hours, that's a long time to allow a house to cool off during the winter.  Even if you push the heat up a few degrees before the rate spike, I don't think your house is going to be comfortable after 5 hours without the heat kicking on.

As for making your wife get up early to avoid the rate spikes, my wife isn't a morning person, if I tried to make her get up early to save a few bucks, I'd have to enter the witness protection program to continue living.

Around here, the on/off peaks rates are about $.12 and $.06. But the on peak time is 10am through 7pm. So you need some type of active storage (like a water tank) to make use of it. But the payback is quite attractive.

Posted By jonr on 11 May 2010 11:54 AM
... the on peak time is 10am through 7pm. So you need some
type of active storage (like a water tank) to make use of it.

Even with active storage, a 9-hour peak time window sounds like a real
challenge. That gives you 15 hours to re-charge the storage tank while
simultaneously heating the house -- during the coldest 15 hrs of the day.

I believe that approximately doubles your required tonnage. Ouch!